The effect of addition of helium gas to the following reaction in equilibrium state at constant volume, is :
${\mathrm{PCI}}_5( \mathrm{g})\rightleftharpoons {\mathrm{PCl}}_3( \mathrm{g})+{\mathrm{Cl}}_2( \mathrm{g})$

${\mathrm{PCI}}_5( \mathrm{g})\rightleftharpoons {\mathrm{PCl}}_3( \mathrm{g})+{\mathrm{Cl}}_2( \mathrm{g})$
In the above reaction vessel, when helium(an inert gas) is added following cases can happen for isobaric and isochoric processes respectively:

Case 1 : At constant P - volume will increase so reaction will shift in forward direction then answer will be A

Case 2 : At constant volume no change in active mass so reaction will not shift in any direction then answer will be D.

Thus, at constant volume, addition of helium will not have any impact on the equilibrium.